Most hydrocarbon wells are drilled in successively lower casing sections, with a selected size casing run in a drilled section prior to drilling the next lower smaller diameter section of the well, then running in a reduced diameter casing size in the lower section of the well. The depth of each drilled section is thus a function of (1) the operator's desire to continue drilling as deep as possible prior to stopping the drilling operation and inserting the casing in the drilled section, (2) the risk that upper formations will be damaged by high pressure fluid required to obtain the desired well balance and downhole fluid pressure at greater depths, and (3) the risk that a portion of the drilled well may collapse or otherwise prevent the casing from being run in the well, or that the casing will become stuck in the well or otherwise practically be prevented from being run to the desired depth in a well.
To avoid the above problems, various techniques for drilling a well with casing have been proposed. This technique inherently runs the casing in the well with the bottom hole assembly (BHA) as the well, or a section of the well, is being drilled. U.S. Pat. Nos. 3,552,509 and 3,661,218 disclose drilling with rotary casing techniques. U.S. Pat. No. 5,168,942 discloses one technique for drilling a well with casing, with the bottom hole assembly including the capability of sensing the resistivity of the drilled formation. U.S. Pat. No. 5,197,533 also discloses a technique for drilling a well with casing. U.S. Pat. No. 5,271,472 discloses yet another technique for drilling the well with casing, and specifically discloses using a reamer to drill a portion of the well with a diameter greater than the OD of the casing. U.S. Pat. No. 5,472,051 discloses drilling a well with casing, with a bottom hole assembly including a drill motor for rotating the bit, thereby allowing the operator at the surface to (a) rotate the casing and thereby rotate the bit, or (b) rotate the bit with fluid transmitted through the drill motor and to the bit. Still another option is to rotate the casing at the surface and simultaneously power the drill motor to rotate the bit. U.S. Pat. No. 6,118,531 discloses a casing drilling technique which utilizes a mud motor at the end of coiled tubing to rotate the bit. SPE papers 52789, 62780, and 67731 discuss the commercial advantages of casing drilling in terms of lower well costs and improved drilling processes.
Problems have nevertheless limited the acceptance of casing drilling operations, including the cost of casing capable of transmitting high torque from the surface to the bit, high losses between the surface applied torque and the torque on the bit, high casing wear, and difficulties associated with retrieving the bit and the drill motor to the surface through the casing.
The disadvantages of the prior art are overcome by the present invention, and improved methods of casing drilling are hereinafter disclosed which will result in a casing run in a well during a casing drilling operation, with lower costs and improved well quality providing for lower cost and/or enhanced hydrocarbon recovery.